1. Field of the Invention
The present invention relates to a signal processing apparatus and signal processing method and, more particularly, to an apparatus and method suitably used in a digital audio instrument that demodulates an input digital signal and outputs an analog signal.
2. Description of the Related Art
Conventionally, consumer digital audio instruments include a CD (Compact Disc) player, MD (Mini Disc) player, DAT (Digital Audio Tape) player, BS (Broadcasting Satellite)/CS (Communication Satellite) broadcast tuner, and the like.
FIGS. 14A to 14E show examples of the arrangements of the conventional digital audio instruments.
Digital audio data output from each of pre-stage circuits and the like indicated by FIGS. 14A to 14D undergoes predetermined processes by a post-stage circuit indicated by FIG. 14E and is demodulated into an analog signal, thus outputting the analog signal.
FIG. 14A indicates an example of an instrument having a digital input terminal, and a digital signal is input from a digital input terminal 1401 (e.g., an input terminal complying with the S/PDIF (Sony Philips digital interface) standard or the like). The input digital signal is output via a digital audio interface receiver IC (integrated circuit) 1402 (to be referred to as a “DIR IC” hereinafter) as an interface conversion IC provided to connect the post-stage circuit indicated by FIG. 14E.
FIG. 14B indicates an example of a CD player or MD player, and a signal read out from a recording medium 1403 such as a CD, MD, or the like using a laser beam is amplified by an RF amplifier 1404 (to be referred to as an “RF AMP” hereinafter). Furthermore, the amplified signal undergoes a predetermined process by a signal processing IC 1405, and is then output to the post-stage circuit.
FIG. 14C indicates an example of a BS/CS broadcast tuner, and a signal, which is transmitted as a broadcast wave and is received by an antenna 1406, is input to and demodulated by a tuner 1407. Furthermore, the demodulated signal is decoded by a decoder IC 1408, and then undergoes a predetermined process by a signal processing IC 1409. The processed signal is then output to the post-stage circuit.
FIG. 14D indicates an example of an audio input, and an audio signal input via an audio input instrument 1410 such as a microphone or the like is processed by an amplifier & analog low-pass filter (LPF) 1411. The processed signal is converted into digital data by an A/D converter 1412, and the digital data is output to the post-stage circuit.
Each digital audio data output as described above undergoes a filtering process, i.e., is up-sampled by a digital filter 1413 in the post-stage circuit indicated by FIG. 14E. Note that the cutoff frequency of the digital filter 1413 is ½ the sampling frequency of input digital data according to the sampling theorem.
Furthermore, the digital audio data that has undergone the filtering process by the digital filter 1413 is converted into an analog audio signal by a D/A converter 1414, and an analog LPF 1415 removes, from that analog signal, noise (quantization noise or RF noise) generated upon conversion from the digital signal to the analog signal by the D/A converter 1414. The analog signal is then output from an analog output terminal 1416.
FIG. 15 shows the sampling frequencies of the aforementioned conventional digital audio instruments, and the sampling frequency of the CD or MD player is 44.1 kHz. The sampling frequency of the BS/CS broadcast tuner is 32 kHz or 48 kHz, and that of the DAT player is 32 kHz, 44.1 kHz, or 48 kHz.
In this manner, the sampling frequency of the conventional digital audio instrument is one of 32 kHz, 44.1 kHz, and 48 kHz. For this reason, each of the conventional digital audio instruments shown in FIGS. 14A to 14E can reproduce an analog signal without data loss from input digital data when the cutoff frequency of the analog LPF 1415 at a final stage of the post-stage circuit is set at 24 kHz or higher, which is ½ of 48 kHz as the highest one of the three different sampling frequency, on the basis of the sampling theorem.
For example, in a practical product, the cutoff frequency of a digital filter is ½ the sampling frequency of a connected instrument (a circuit for inputting digital data) (for example, in case of a CD or MD player with the sampling frequency=44.1 kHz, the cutoff frequency=22.05 kHz; in case of a BS/CS broadcast tuner with the sampling frequency=32 kHz, the cutoff frequency=16 kHz). The analog LPF uses an operational amplifier, and is normally designed to have a cutoff frequency that assumes a fixed value within the range from 30 to 50 kHz.
In this manner, in the conventional digital audio instruments, the sampling frequency of input digital data falls within the range from 32 to 48 kHz, and does not have large differences depending on digital data. For this reason, in the conventional digital audio instruments, even when the cutoff frequency of filters (digital filter, analog LPF) is fixed at a predetermined value, an output analog signal upon reproduction does not suffer any serious problem.
Recently, a DVD-Video player with a sampling frequency of 96 kHz, a sound board for a personal computer, which can input/output data in conformity to the S/PDIF standard or the like with a sampling frequency of 96 kHz, and a DVD-Audio player with a sampling frequency of 192 kHz have been commercially available. As described above, the sampling frequency of consumer digital audio instruments falls within the range from 32 kHz to 192 kHz, and the sampling frequency range has largely broadened.
However, although digital data corresponding to the broadened sampling frequency range from 32 kHz to 192 kHz can be input (can undergo a reproduction process), if the cutoff frequency of the analog LPF arranged at the final stage is set at a fixed value falling within the range from 30 to 50 kHz as in the conventional instruments, when a medium (e.g., DVD-Audio) that records digital data with a sampling frequency of 192 kHz is reproduced, a signal up to 96 kHz can be reproduced according to the sampling theorem, but a reproduced signal beyond the range from 30 kHz to 50 kHz is lost in practice.
On the other hand, if the cutoff frequency of the analog LPF is set near 100 kHz about ½ of 192 kHz as the sampling frequency, when digital data with a sampling frequency of 32 kHz (e.g., BS broadcast tuner) is reproduced, frequency components within the range from 16 to 100 kHz as an unnecessary frequency band that include noise generated by, e.g., a D/A converter (components of such unnecessary frequency band upon reproduction will be referred to as “out-of-band noise components” hereinafter) are also reproduced as noise components, as shown in FIG. 16B.
FIGS. 16A and 16B show the frequency bands of reproduced signals output from digital audio instruments upon reproducing digital data with a sampling frequency of 32 kHz. FIG. 16A shows the frequency band of a reproduced signal by an instrument which is incompatible to digital data with a sampling frequency of 96 kHz or 192 kHz, and in which a cutoff frequency fc(L) of an LPF is 30 kHz. On the other hand, FIG. 16B shows the frequency band of a reproduced signal by an instrument which is compatible to digital data with a sampling frequency of 96 kHz or 192 kHz, and in which a cutoff frequency fc(H) of an LPF is 100 kHz.
As shown in FIGS. 16A and 16B, the reproduced signal output from the instrument in which the cutoff frequency of the LPF is 30 kHz has a frequency band from 0 to 30 kHz, and the reproduced signal output from the instrument in which the cutoff frequency of the LPF is 100 kHz has a frequency band from 0 to 100 kHz. However, a region required to reproduce digital data with a sampling frequency of 32 kHz is a region SB1 from 0 to 16 kHz according to the sampling theorem. Hence, components NB1 and NB2 with frequencies higher than 16 kHz in the reproduced signals are out-of-band noise components, and are reproduced as noise in the reproduced signals.
As can be seen from FIGS. 16A and 16B, the instrument in which the cutoff frequency of the LPF is 100 kHz has a larger area of out-of-band noise components than that of the instrument in which the cutoff frequency of the LPF is 30 kHz, and the S/N (signal/noise) ratio and THD+N (noise+distortion ratio) characteristics deteriorate. For this reason, upon simply comparing numerical values of performance, the performance of the instrument which is compatible to digital data with a sampling frequency of 96 kHz or 192 kHz is worse than that of the incompatible instrument.